This invention relates to hot plates for cooking and particularly the type comprising a shallow sheet metal cup containing a ceramic support positioning a horizontal flat spiral of electric resistance heating wire which when supplied with current produces heat. Such hot plates are used under the bottoms of the glass plates of glass top cooking ranges.
Hot plates particularly when installed in glass top ranges have used metal alloy electric resistance wire as a heating element. The metal alloy wire is elastic at room temperature and is not easily damaged by rough handling of the hot plate such as occurs during shipment and installation in the range. On the other hand, it is slow to heat and cool and operates at low efficiency because the heat is transmitted to cooking utensils on the glass top largely by convection to the glass. Metal alloy resistance heating wire operates at usual temperatures between 1500.degree. F. and 2100.degree. F. and has a maximum operating temperature of 2200.degree. F. At these temperatures the metal wire heat radiation is low. Public reaction against the low efficiency and slow heating and cooling of glass top electric ranges have resulted in some loss of their initial popularity.
Recent development of ceramic glass with a high transmittance of infrared radiation in the range of 0.6 to 4.5 microns wavelength has not helped in solving the above indicated problem, because at the operating temperature range of metal alloy resistance wire there is only a small amount of heat radiation provided for transmittance through the high transmittance glass if it is applied as a glass top range installation.
The high transmittance glass would have a great advantage if the heating element could be operated at the high temperatures where the heat radiation becomes substantial. For example, at 2800.degree. F. the wire heat radiation would become more than twice that at the 2200.degree. F. maximum operating temperature of metal alloy wire. The heat radiation at 2200.degree. F. is only 44.3% of that obtained at 2800.degree. F., and at the more usual metal wire operating temperature of 1500.degree. F. the heat radiation is only 13.1% of that provided at 2800.degree. F.
If the metal alloy wire is replaced by molybdenum disilicide (MoSi.sub.2) wire the operating temperature of 2800.degree. F. becomes possible without requiring protection from the ambient atmosphere. The prior art has made efforts to develop a hot plate using MoSi.sub.2 wire as the heating element, because of its apparent advantages.
Unfortunately, at room temperature MoSi.sub.2 wire is glasslike and very brittle and is easily broken by the shock of rough handling such as occurs during hot plate shipping and installation in a glass top range. This has so far prevented commercial production of hot plates using MoSi.sub.2 heating elements.
The object of the present invention is to provide a hot plate using a MoSi.sub.2 wire heating element and which solves the prior art wire breakage problem that has prevented commercialization of MoSi.sub.2 hot plates.